The present invention relates to method and system for carrying out magnetic resonance imaging and, more particularly, relates to a magnetic resonance imaging method and magnetic resonance imaging apparatus in which a magnetization transfer contrast pulse is used to acquire magnetic resonance angiography image data.
Magnetic resonance (MR) angiography is a well-known imaging method in a medical field, for example, as means for imaging blood vessels, such as those in the head (brain) of an examinee or patient. A technique has recently been developed that includes applying a method called MTC (Magnetization Transfer Contrast) to the MR angiography to improve delineation capabilities for small vessels by increasing the signal intensities for the blood vessels in relation to those of the parenchyma.
Namely, the Magnetization. Transfer Contrast (MTC) method has proven to be a powerful method to increase contrast in magnetic resonance imaging (MRI) and angiography (MRA), using the relaxation difference in tissues. The magnetization transfer technique, originally described by Forsen and Hoffman, was first employed in imaging using continuous wave off-resonance irradiation. A pulsed method, using an on-resonance binominal pulse, was designed to obtain MTC in many imaging sequences and has been applied in angiography. The MTC effects on brain background as well as on stationary, freshly drawn, heparinized vanous blood were examined using a pulsed method and an off-resonance Gaussian RF pulse. Improved brain background suppression using the MTC pulse was reported. However, the reduction of the blood signal was too severe to be neglected in such a method.
FIG. 6 shows the spectra of protons in water and in macromolecules.
As can be seen from this figure, a proton contained in water has a resonant frequency of about 64 MHz, for example, under a magnetic field of 1.5 tesla (T). A proton contained in macromolecules, on the other hand, is characterized by its wide frequency bandwidth.
Here, prior to execution of the MR angiography sequence, if a proton in water is selectively excited by means of an RF (radio-frequency) pulse called MTC pulse, and the frequency is shifted for example by 500 Hz from the resonant frequency, the signal level from the proton in the macromolecules is reduced as indicated by the dashed line. In addition, the MR signal level from protons contained in water is also reduced as indicated by the dashed line.
This signal level reduction presumably results from the crossover relaxation or exchange of protons in water with the protons in the macromolecules. By utilizing this MTC effect, an image having a contrast different from conventional ones may be obtained in accordance with the ratio by which the macromolecules exist. Furthermore, since there is also an effect of greatly reduced signal level of parenchyma portions in comparison to the signal level of blood vessel portions, the MTC technique is applied to angiography for depicting small blood vessels.
FIG. 7 is a brief sectional view showing a portion of an MRI (magnetic resonance imaging) apparatus, where an RF coil 10 to be used for both transmitting and receiving of signals is disposed around the head 9 of an examinee 8 such as patient.
In the case of employing such MRI apparatus, an MTC pulse is first transmitted from the RF coil 10 prior to execution of a pulse sequence for the MR angiography and, then, the MR angiography sequence such as the FE (field echo) method is applied.
A problem occurs, however, in the case of using a whole body type RF coil.
FIG. 8 shows an MRI apparatus having a whole body coil, where a whole-body coil 11 for transmitting only is disposed entirely around the examinee 8 and a head RF coil 12 for receiving only is only disposed around the head 9 of the examinee 8.
In such an MRI apparatus, the transmitting range of the MTC pulse to be transmitted from the whole body coil 11 extends along the length of the examinee from where the blood flows (heart side) into the head 9.
Therefore, for this reason, the blood flowing out of the heart flows into the head while the MTC pulse is applied repeatedly for many times, and a state of saturation of the flowing blood occurs in the sequence of the MR angiography which is to be subsequently performed.
Thus, even if MR angiography is applied, the signal level of the flowing blood becomes smaller and the S/N (signal/noise) ratio related to depiction of the blood vessels is extremely worsened.
For this reason, the MTC pulse is not always applied to the MR angiography in a conventional MRI apparatus of the whole-body coil type.